I have these red marks on my thighs.?

It sounds as though you have stretch marks. While some us have a generic predisposition to stretch marks and get lots of them other people will never develop any. Some reports say they are linked to weight gain. They are nothing to worry about but you can try putting vitamin E cream on them and mage to reduce them. Pregnancy is a time when many women experience them especially on the thighs, and stomach. Eat healthy, exercise regularly and try not to gain too much weight and you will reduce the amount coming your way.